<< i guess xp is awesome... if you havent tried OSX or Linux. >>
I've run ...
ms dos (a few versions, with a few versions of things like xtree, xtree gold, etc)
macos (8,9,10)
win3.1
win3.11
win95
win98
win2k
winNT 4
winXP
linux (debian, redhat, and corel .. i wish corel would have made it)
freebsd
openbsd
netbsd
a variety of microkernels
And I can say, without a doubt, that linux is not in the top 4.
In my not so humble opinion, for the desktop I'd rank:
Win2k, OSX, WinXP, Freebsd
For servers:
FreeBSD (only above openbsd for smp support), OpenBSD, NetBSD, OSX/Win2k (tossup, both have their advantages)
note that linux is missing on both lists.